How Electron Transport Is Used to Trap Chemical Bond Energy

Cellular RespirationHow Electron Transport Is
Used to Trap Chemical
Bond Energy
Having seen that ATP is the one common
energy denominator by which all
cellular machines are powered, we are
in a position to ask how this energy is
captured from fuel substrates. This
question directs us to an important
generalization: all cells obtain their chemical energy requirements from
oxidation-reduction reactions. This
means that in the degradation of fuel
molecules, hydrogen atoms (electrons
and protons) are passed from electron
donors to electron acceptors with a
release of energy. A portion of this
energy is trapped and used to form the
high-energy bonds in ATP.

Figure 4-9 A redox pair. The
molecule at left is oxidized by
the
loss of an electron. The molecule at
right is
reduced by gaining an
electron.

Because they are so important, let
us review what we mean by oxidationreduction
(“redox”) reactions. In these
reactions there is a transfer of electrons
from an electron donor (the reducing
agent) to an electron acceptor (the oxidizing
agent). As soon as the electron
donor loses its electrons, it becomes
oxidized. As soon as the electron
acceptor accepts electrons, it becomes
reduced (Figure 4-9). In other words,
a reducing agent becomes oxidized
when it reduces another compound,
and an oxidizing agent becomes
reduced when it oxidizes another compound.
Thus for every oxidation there
must be a corresponding reduction.

In an oxidation-reduction reaction
the electron donor and electron acceptor
form a redox pair:

When electrons are accepted by the
oxidizing agent, energy is liberated
because the electrons move to a more
stable position. In a cell, electrons flow
through a series of carriers. Each carrier
is reduced by accepting electrons
and then is reoxidized by passing
electrons to the next carrier in the
series. By transferring electrons stepwise
in this manner, energy is gradually
released, and a maximum yield of
ATP is realized.